Unraveling the Unique Behavior of Ce- and Ni- substituted Barium Ferrite in Reversible Protonic Cells: A Fundamental Study Bridging Experiments and Computations

• Applicant: Professor Dr. Francesco Ciucci
• Subject Area: Synthesis and Properties of Functional Materials ; Computer-Aided Design of Materials and Simulation of Materials Behaviour from Atomic to Microscopic Scale
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537578713

A shift towards sustainable energy demands innovative conversion and storage technologies. Reversible solid oxide cells hold great promise for this application; as fuel-flexible fuel cells, they generate electricity with record-high thermodynamic efficiencies and, as electrolyzers, they store electrical energy as a fuel, such as hydrogen or syngas. However, slow oxygen conduction within the electrolyte requires solid oxide cells to operate between 800 and 1000 °C, which increases system and materials costs. Reversible protonic ceramic cells (RePCCs) with proton-conducting electrolytes can operate at 400–600 °C, lowering costs. However, RePCCs are hindered by sluggish air electrode reaction kinetics, necessitating novel, high-performance air electrodes. Preliminary experiments have shown that RePCCs with Ba10/11Ce1/11Fe10/11O3-δ air electrodes achieve high performance and stability; a surprising result given the inefficacy of Fe and Ce as oxygen reaction catalytic centers. Furthermore, Ba10/11Ce1/11Fe10/11O3-δ avoids the formation of reaction-blocking compounds, which typically degrade performance. Interestingly, Ba10/11Ce1/11Fe10/11O3-δ exhibits a unique Janus-type Ce substitution, with Ce found in both Ba and Fe sites. Janus-type substitution results in an unexplored interplay of compressive and tensile strain (due to Ce’s presence in the Ba and Fe sites, respectively), whose implications on electrocatalytic activity and stability are unknown. Performance is further enhanced after introducing Ni to form Ba9/11Ce1/11Fe8/11Ni2/11O3-δ, which results in Ce’s exclusive presence in the Fe site. Yet, the mechanisms behind Ce’s location in the Fe site and the interplay between Ni and Ce are unknown. Additionally, it is unknown how tensile strain induced by Ce in the Fe site impacts stability and reactivity. Unraveling these underlying mechanisms is crucial for the future development of air electrodes. To address these questions, the mechanisms influencing electrocatalytic performance and stability in Ba9/11Ce1/11Fe(10-x)/11Nix/11O3-δ (x=0, 1, 2, 3) will be investigated with conventional characterization and advanced synchrotron-based experiments. Furthermore, epitaxial thin films will provide mechanistic insights without the uncertainties of conventional powder samples. Ab initio simulations will complement experimental analysis by unraveling the reaction mechanisms and surface stability. Finally, the resulting computational and experimental knowledge will guide the development of a high-performance RePCC.

• DFG Programme: Research Grants
• International Connection: South Korea
• Cooperation Partners: Professor WooChul Jung; Professor Jongwoo Lim